1. Field of the Invention:
The present invention relates to a radiative semiconductor device, and more particularly to a light-emitting diode having a hetero-junction for radiating incoherent light.
2. Description of the Prior Art:
Radiative semiconductor devices are usually formed with compound or compound mixed-crystal semiconductors. Manufacturing processes impose various limitations to the structure of the semiconductor devices. Formation of a doped region is usually done by diffusion or liquid phase epitaxial growth.
Now, some typical examples of conventional light-emitting diodes will be described hereunder.
A GaAsP light-emitting diode is formed with an n.sup.+ type GaAs substrate, an n type GaAsP epitaxial layer formed on the n.sup.+ type GaAs substrate, and a p type region which is selectively diffused in the n type GaAsP layer. The p type diffusion region has a typical impurity concentration of about 1.times.10.sup.18 cm.sup.-3 and is formed to be thin, being below several micrometers, to avoid excessive absorption of propagated light in this p type region. The output light is extracted from that side of this p type region located opposite to the substrate.
A GaP light-emitting diode is formed with an n.sup.+ type GaP substrate, an n type epitaxial GaP layer and a p type epitaxial GaP layer, these latter two epitaxial layers being successively grown on said n.sup.+ type GaP substrate. The p type layer has a typical impurity concentration of about 1.times.10.sup.18 cm.sup.-3. The output light is extracted at the side of this p type layer.
A GaAs light-emitting diode has a structure similar to that of the above-mentioned GaP light-emitting diode. The p type surface layer has a typical impurity concentration of about 2.times.10.sup.18 cm.sup.-3. The output light is extracted from at either the side of the p type surface layer or at the side of the n.sup.+ type substrate (in this latter case, the substrate is partially etched away at sites where the output light is to be extracted).
A GaAlAs light-emitting diode is formed with a p.sup.+ type GaAs substrate, a p type Ga.sub.1-x Al.sub.x As epitaxial layer formed on the GaAs substrate, and an n type Ga.sub.1-y Al.sub.y As epitaxial layer formed on the p type epitaxial layer. The compositions (mixing ratios) x and y are selected to satisfy the condition x&lt;y, to effectively extract the output light at the said of the n type epitaxial layer. Such a light emitting diode is described in U.S. Pat. No. 4,296,425, issued to Nishizawa on Oct. 20, 1981. The Nishizawa patent does not specify the carrier concentrations of the respective regions thereof. However, in practice, the n type epitaxial layer has a typical impurity concentration of about 1.times.10.sup.17 cm.sup.-3. The present invention is an improvement of the invention of the Nishizawa patent. As is hereinafter explained, in accordance with the present invention, selecting the carrier concentrations, and compositions in a specific predetermined relationship, provides a very high brightness diode.
It should be understood that there exist several types of structure for the light-emitting diode, and the different considerations should be paid for different structures.
Usually in group III-V compound semiconductor light-emitting diodes, it is easier to obtain better light emission efficiencies in p type regions than in n type regions.
Light-emitting diodes utilize spontaneous emission of radiation (in contrast to stimulated emission of radiation in semiconductor lasers). Therefore, minority carrier lifetime in a light-emitting diode is relatively long. Minority carriers injected into a radiative region across a pn junction have a large possibility of being captured by non-radiative recombination centers and of being recombined with majority carriers thereat without radiating light rays. Thus, it is very important to reduce non-radiative recombination centers in the radiative region of a light-emitting diode.
Lattice defects are largely responsible for the generation of non-radiative recombination centers. Usually, one of the p type and n type regions constituting a pn junction has a smaller density of non-radiative recombination centers, and forms a main radiative region. In GaAs and Ga.sub.x Al.sub.1-x As, defects are more likely to be formed in an n type region than in a p type region. In an n type region doped with a donor impurity such as Te, Se or S, vacancies may be formed which electrically compensate for the donor atoms. The density of such vacancies is considered to be proportional to the doped concentration. Such vacancies and/or the combinations of vacancy and donor impurity are considered to be very effective non-radiative centers.
In a light-emitting diode having a diffused pn homo-junction, usually zinc(Zn) is diffused as an acceptor impurity into an n type crystal. It is very difficult to diffuse a donor impurity in a p type crystal, because there is no donor impurity which has a diffusion constant as large that of zinc.
Diffusion of zinc naturally makes the carrier concentration in the p type region higher than that in the n type region. The concentration of diffused zinc should not be made too high in order to keep the crystal properties of the diffused p type region good. The n type region into which zinc is diffused should have a lower impurity concentration than that in the p type diffusion. Hence, it is difficult to raise the injection of minority carriers into the radiative region across the pn junction.
Light-emitting diodes manufactured by liquid phase epitaxy generally exhibit higher light emission efficiencies than those of diffused light-emitting diodes. Epitaxial layers have lower defect densities than diffused regions. Furthermore, any combination of impurity concentrations can be selected for an epitaxially grown pn junction. Therefore, in an epitaxial pn homo-junction diode, carrier concentrations of the respective regions are selected to maximize the injection efficiency of minority carriers into the radiative region. For example, in a light-emitting diode having a p type radiative region, the carrier concentration of the p type region is selected sufficiently low as compared with that of the n type region to raise the electron injection into the p type region as compared with the hole injection into the n type region. In any case, the radiative region is arranged to have a low impurity concentration for raising the injection efficiency and the light emitting efficiency.
The hetero-junction light-emitting diode has the advantage of providing a window effect as described above, and can have a higher external efficiency than the homo-junction light-emitting diode.
The hetero-junction diode can have further advantages over the homo-junction light-emitting diode, but its superiorities have not been fully exploited.